RU2357870C1 - Method and system for laser marking precious stones, such as diamonds - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: present invention relates to the method and system for laser marking precious stones and, particularly to the method and system for engraving authentication codes. In the system for laser marking precious stones such as diamonds, marks consist of several microscopic dots, increase of which can be initiated upon effect on natural internal defects or impurities inside the precious stone of a strictly focused laser pulse sequence. The marks are inscribed by laser pulses, carrying significantly less energy than threshold energy required for inscription inside ideal material of precious stone. The method of laser marking and encryption takes into account random spatial distribution of defects, present in natural precious stones, as well as their much localised character. Authentication data are encrypted in the precious stone in the relative spatial arrangement of dots which form a mark. Dots, engraved under the surface of the precious stone, can be made undetectable to the naked eye and a magnifier through limiting their individual size to several micrometres. The mark can be detected using a special optical reading device.

EFFECT: laser inscription of permanent point marks inside precious stones.

40 cl, 14 dwg

Description

FIELD OF THE INVENTION

The present invention relates, in General, to a method and system for laser marking of precious stones and, in particular, to a method and system for engraving authentication codes consisting of several microscopic point marks created by affecting localized internal defects in the volume of a gemstone controlled by a sequence of laser pulses.

BACKGROUND OF THE INVENTION

Tagging Gemstones

Pre-inscribing a uniquely identified identification tag or indicium (lat.) On a gem that is stolen, lost or mixed in a heap helps to identify it in case of vindication (legal reclamation) and subsequent return to its rightful owner. Therefore, insurance companies urgently need to tag gems of great value, since many of these products are insured. On the other hand, inscribing indicium (hereinafter “the mark”), which simply indicates the place of extraction or country of origin of precious stones such as diamonds, will help to effectively prevent the so-called “conflict diamonds” from entering legally operating diamond processing enterprises.

The labeling of products of various nature in order to uniquely identify, classify, track or ease of vindication is already well developed.

Labeled characters can take the form of human-readable codes, such as logos, artwork, samples, or serial numbers, from a sequence of alphanumeric characters.

You can enter machine-readable codes, such as ordinary one-dimensional bar codes or two-dimensional arrays of point labels, designed in accordance with various types of symbols. Some of the distinguishing features of gemstones make their marking very difficult. For example, signs have to be engraved on the surface of very small products, which usually have a large number of even smaller faces (facets) oriented in different directions. In addition, if the gemstone is installed in the frame, only a limited part of the outer surface of the stone is available for tagging. These difficulties are compounded by the fact that gemstones, such as diamonds, are very high hardness materials, prone to cracking under severe stress or excessive local heating. And more importantly, inscribing a permanent sign on a faceted and polished gemstone in no way should impair its appearance, reduce quality and monetary value.

Laser marking of gemstone marks

Among the various methods developed for the permanent labeling of precious stones, laser marking has long been known in the precious stone processing industry. One of the preferred methods of laser marking is based on the use of a laser beam with the appropriate characteristics, and the beam is directed to part of the polished surface of the gem. Some key characteristics of the beam, such as the average power or energy of each pulse, the focusing conditions, the wavelength and the duration of the laser exposure, are selected so as to ablate a thin layer of surface material. For laser marking, various types of laser systems have been proposed and used. For example, U.S. Patent Nos. 5149938, 5410125 and 5573684 (all issued to Winston et al.), 6187213 to Smith, 6483073, 6593543, 6747242 and 6788714 (all to Bendery) describe the use of excimer lasers, capable of transmitting ultraviolet laser radiation, i.e. laser radiation with a wavelength of less than about 400 nm (nm - nanometer, 1 nm = 10 ^-9 m). Shorter wavelengths are preferred because the diameter of the engraved sections and the width of the engraved linear segments correlate with the wavelength of the beam. It should be noted that most native diamonds are of type Ia. Their edge of the absorption band of ultraviolet radiation corresponds to a wavelength of about 291 nm, so that they are practically transparent for wavelengths in the visible region of the spectrum - in the range from 400 to about 700 nm. However, semiconductor laser systems have proven attractive as laser sources for labeling gemstones, especially if their primary output beam is doubled in frequency to obtain the final wavelength, usually in the range of 500 to 600 nm in the visible spectrum. The use of Nd: YAG lasers for engraving on the surface of gemstones is described in US Pat. Nos. 4,392,476 (issued to Gresser et al.), 4,467,172 (issued to Ehrenwald and others), 5,753,887 (issued to Rosenwasser and others) and 6713715 (issued to Christensen and others), and the use of Nd: YLF lasers is disclosed in US patents 5932119, 6211484, 6476351 and 6684663 (all issued by Kaplan and others). Laser beams having a cross-sectional area of sufficient size, when hit on the surface of the product, can create patterns of complex shapes by ablation by using a mask in which cutouts are mechanically made to accurately reproduce the shape of the desired pattern. Alternatively, signs having complex patterns can be engraved with a laser beam that is very precisely focused on a very small area on the surface of the product. For this purpose, the product can be installed with an electric drive table for moving in three directions (XYZ) with pre-programmed movements. Another approach is to use a beam control device for controllably scanning a laser beam over a limited surface of an article that is held stationary. Even with very precise focusing, the average power or energy of each pulse supplied by the laser source may not be sufficient for the threshold of ablation of the surface of gemstones such as diamond, which are very high hardness materials and usually transparent. In this case, before exposure to the laser beam, a light-absorbing material such as paint or labeling paste can be applied to the surface of the product. An alternative to using light-absorbing coatings is the use of a pulsed laser that can emit laser pulses of less than about 1 ns duration (ns is a nanosecond, 1 ns = 10 ^-9 s) to lower the threshold energy for the evaporation of most materials, as described in US Pat. No. 6,713,715 (issued to Christensen) and etc.).

Signs engraved using the conventional method described in the above opposed patents do not impair the appearance and sorting of the gemstones, since these marks are usually engraved on a part of the surface of the girdle (also called girdle) of the gemstones. In particular, these marks engraved on diamonds often create some darkening due to the growth of the surface layer of graphite during laser ablation. In many cases, the presence of graphite is an insignificant problem, and, in fact, it can help provide better visibility of signs if they are designed to be read with a small magnifier. If necessary, this layer of graphite can be removed by surface treatment. An example of such processing is given in US Pat. No. 4,467,172 (issued to Ehrenuald et al.); it consists in applying a temperature of + 700 ° C in combination with hydrochloric (hydrochloric) acid. In addition to the high-contrast appearance of the signs created by the presence of a graphite layer on the engraved parts of the surface, any sign can be made easier to detect and recognize simply by increasing it. The advantage of inscribing easily visible signs that are large enough is that in some specific cases they can serve as effective deterrents against theft.

Unfortunately, visible signs inscribed directly on the surface of precious stones can be easily faked by simply polishing the engraved part of the surface of the girdle or using other types of surface treatments. After this operation, perhaps, a marking of a new but illegal sign will follow. Surface treatment in order to remove the sign engraved on the surface of the gemstone will consist, for example, of removing any trace of graphite in the engraved pattern (if any) and then filling the engraved areas with a certain type of product to fill cracks or fractures, well known in this field . Even if the marking on a part of the surface of the belt does not impair the appearance and sorting of the gemstone, the mark inscribed on the girdle may become hidden if the marking is transferred to the gemstone before it is mounted in the frame. Many frames have grips that prevent visual access to the entire surface of the girdle.

On the other hand, in some other cases, the identification may be required to be as low as possible to avoid unauthorized detection. The obvious way to achieve this goal is to inscribe signs with very small overall dimensions. As already noted, the size of the smallest details that can be entered by a laser beam focused by conventional optics is essentially limited by the wavelength of light reaching what is called the diffraction limit of light. Unfortunately, powerful, reliable and relatively inexpensive lasers emitting at wavelengths shorter than about 190 nm and structurally designed for industrial applications are still missing.

Significant improvement of the existing methods of laser marking on the surface of precious stones was carried out using a special method known as near-field optics. U.S. Patent 6,624,385, U.S. Patent Application 10/607184 and U.S. Patent Application 10/607185 (inventors of Patton et al.) Describe the use of near-field optics for marking gemstones with a wide variety of lasers, such as excimer lasers and Nd: YAG lasers with double frequency. This method allows you to enter "microsigns" consisting of elements whose dimensions are much smaller than the optical diffraction limit allows. Near-field optics can be realized by supplying a laser beam through tapering optical fibers or, preferably, using a solid immersion lens, the flat exit surface of which is installed in close contact with part of the surface of the gemstone.

In addition to the known disadvantages of laser marking on the surface of precious stones, marking micro-signs of very small sizes can make it difficult to find them within a reasonable time. Usually a search key should be provided, or microsigns should fit in exact places relative to some noticeable landmarks on the stone, such as the geometric center of the plate (the flat part of the diamond-cut crown). In addition, the reading of subtle micro-signs is usually carried out using complex and expensive devices. Finally, to remove traces of any inconspicuous micro-sign, the counterfeit can easily re-polish the entire outer surface of the stolen gem.

Laser marking of signs in the volume of transparent materials

Regardless of the overall size and complexity, the mark can be made very difficult, if not impossible at all, to counterfeit, if it is engraved far enough from the surface of the gem, leaving the outer surface unchanged by the marking process. In this case, the layer of material between the sign and the outer surface serves as a thick protective barrier, so changing the sign without causing major and irreparable damage to the product marked in this way becomes very difficult. For the labeling of products properties whose dimensions and uses are radically different from those of ordinary gemstones, methods have been developed for subsurface labeling with a laser beam. For example, US Pat. No. 5,206,496 to Clement et al. Describes subsurface laser marking of areas of increased opacity in the body of transparent materials such as glass and plastics. A method for labeling containers is proposed, which is used, for example, for expensive perfumes that are sold in limited quantities in company stores. Labeling in the bulk of the material gives an advantage not only the ability to withstand any surface treatment (including repolishing), but also very difficult for attackers to accurately copy. Laser marking below the surface of diamonds is briefly described in US Pat.

Inserting marks (also called “microstructures”) in the volume of various materials by laser beams is a concept that is promising for incorporating two- or even three-dimensional arrays of densely packed point marks for permanent storage of optical data. This concept is also attractive for constructing optical waveguides used to conduct light into the bulk of optical materials, such as silica glass. For both types of applications mentioned above, the use of a recording laser beam with tightly controlled temporal and spatial characteristics is required to fit microstructures of exact sizes and shapes into the bulk of the transparent material without causing any undesirable damage to the middle of the material or its outer surface. U.S. Patent 5,671,111, issued to Glaser, although mainly related to the storage of optical information, describes the use of ultrashort laser pulses to produce crackless microstructures of regular shape with a high contrast index of refraction in various transparent materials. These materials include quartz glass, plastics, semiconductors, sapphire, and even small crystals and jewelry. The above patent describes three different labeling modes, the first of which provides better control over the shape and size of the inscribed microstructures. This mode is based on the use of a rigidly focused pulsed laser beam with an extremely short pulse duration, i.e. ranging from a few fs (fs is a femtosecond, 1 fs = 10 ^-15 s) to about 200 ps (ps is a picosecond, 1 ps = 10 ^-12 s). Another requirement of this particular labeling regime relates to the energy carried by each laser pulse, which should be comparable (or several times higher) with the threshold energy required to cause permanent structural changes (damage) in the surrounding transparent material at the selected laser wavelength and focus characteristics.

The results of a successful demonstration of this subsurface tagging method are given in the aforementioned patent and journal articles such as E. N. Glezer et al. (Glezer et al.), "Three-dimensional optical storage inside transparent materials" (Optics Letters, Vol.21, p.2023-2025 (1996), and E. N. Glezer et al., "Ultrafast-laser driven micro-explosions in transparent materials", Applied Physics Letters, Vol. 71, p. 882-884 (1997). For example, the authors managed to enter a two-dimensional array of microstructures with a low contrast index of refraction, spaced about 2 μm apart (μm - micrometer, 1 μm = 10 ^-6 m) and having a diameter of 200-250 nm, when viewed from the surface, on which was hit by a laser beam. The microstructures were inscribed at a depth of 100 μm from the surface of the recording medium made of quartz glass. However, it should be noted that the said patent and related journal articles do not report any successful attempt to label in the volume of diamond material. In fact, in these opposed materials, it is only mentioned that the energy threshold for causing structural changes in the volume of diamonds is higher than the energy thresholds of most other transparent materials, not less than 100 times.

Laser marking in the volume of diamonds

Intrigued by the situation just described above not brought to its logical conclusion, and apparently not knowing about US patent 4467172 (issued by Ehrenuald et al.), J. B. Ashcom conducted more systematic experimental studies. aimed at labeling in the body of images of native single-crystal diamonds of type Ia and IIa with laser pulses of a few femtosecond duration. He reported on his main results in Chapter 4 of his doctoral (Ph.D.) dissertation entitled "The Role of Focusing in the Interaction of Femtosecond Laser Pulses with Transparent Materials" (Harvard University of Cambridge, Massachusetts, January 2003). Eshkom noted that the direction of a sequence of femtosecond laser pulses to the same section in a diamond sample can cause optical damage (microstructures) in the sample volume, but only when the laser pulses are focused by a microscope objective having a numerical aperture in the range of about 0.25-0. 45. Eshkom undoubtedly succeeded in tagging microstructures at a depth of about 40 μm below the surface of a diamond sample using laser pulses that transferred energy that varied from about 20 to 90 nJ (nanojoules). One of the important points of his experimental studies is that even with the highest energy level and the largest number of pulses that he used, there were cases when there were no internal damage in the samples of native diamond. In addition, there was a statistically significant component for initiating laser damage from site to site of the same diamond sample, and also from sample to sample. Expressed as a possible reason for this stochastic behavior were spatial changes in the concentration of impurities present in samples of native diamond. A dissertation by another member of the same group (J.C. Hwang, Harvard University, Cambridge, Massachusetts, January 2003) also reports that the microstructures created were dark and opaque , which was hypothetically explained by the presence of graphite and, more likely, the formation of amorphous carbon inside each microstructure. Aware of these results, Eshkom concluded that successful labeling in the volume of diamonds is unlikely.

The decisive role played by impurities and defects in creating marks in the volume of the gemstone material is more clearly indicated by the microphotograph shown in Fig. 1A. Five laser pulses with a duration of approximately 150 fs were focused all in the same volume inside a sample of native diamond. Instead of a single mark centered on the peak of the intensity profile of the focused beam, FIG. 1A shows that at least three distinct marks were created, each of which was outside the volume in which the recording laser beam reached its narrowest transverse spot size. The local optical fluence (integrated optical flux density) at the location of each dark spot visible in this figure was then significantly lower than the maximum fluence of the recording laser beam, but, nevertheless, it was sufficient to initiate structural changes in places where The material contained well localized defects and impurities. On figv presents yet another evidence of a localized nature and random distribution of naturally existing defects and impurities. This figure shows a microphotograph taken on the surface area of a native diamond sample, along which a rigidly focused femtosecond laser beam was transferred along a linear path with a constant speed of 1 mm / s. 50 μJ laser pulses were applied at a frequency of 1 kHz, and the trace shown in this figure travels at a distance of about 2 mm. As can be seen in this microphotograph, the trace inscribed in the volume of this particular sample of native diamond is far from continuous, since it consists of small dark spots with a random distribution along the trajectory. A striking feature of this microphotograph is the presence of a long trace segment located in the central zone of the figure, where there are no dark spots. On the other hand, densely packed dark spots are visible in some places on the left side of the track. In addition, many of these spots are either above or below the trajectory axis, which means that they were formed in places where the local optical fluence of the beam was not at its maximum peak level.

Based on the results presented in figa and 1B, we can conclude that for the successful labeling of microstructures in samples of native diamond, the appropriate choice of pulse energy is important. For example, with excess pulse energy, as was the case shown in FIG. 1A, several off-center marks may form in the material near (and slightly above) the target volume in the material. On the other hand, firing of laser pulses with insufficient energy can lead to failure of tagging in volumes where defects are supposedly absent. Based on the foregoing, it can be expected that suitable pulse energy limits can vary from site to site in the same sample of native diamond, in order to avoid the localized nature and random distribution of defects with which the formation of microstructures begins. In addition, the pulse energy has a great influence on the subsequent growth of inscribed marks. For example, FIG. 1C shows a microphotograph taken on the surface area of a native diamond sample in which a set of marks is inscribed with a sequence of five laser pulses. The energy of each pulse was several microJ in diameter and varied from site to site. The marks visible in FIG. 1C as black zones with irregularly shaped contours were inscribed in a sample of native diamond that was previously faceted to give it a cube shape. The cubic shape allows you to visually observe labels from any flat side wall of the sample, thereby giving accurate information about the distribution of microstructures in a direction parallel to the propagation axis of the recording laser beam. In FIG. 1C, the recording laser beam was incident on the surface of the sample at the top of the figure and propagated parallel to the downward direction of the figure. In this particular example, the length of the microstructures in the vertical direction at the highest energy level used in these tests reaches more than 100 μm, as shown for both marks located on the right extreme part of the figure. As a result, both marks are visible as dark spots with a diameter of approximately 30 μm when viewed from the incidence surface of the sample beam.

It was found that after initiating a structural change with a defect or impurity in the diamond material, the subsequent growth of the mark can be controlled by the correct selection of key parameters of the labeling process, such as pulse energy, the number of laser pulses directed to each region inside the sample, and the characteristics of the recording laser beam. However, the combination of laser parameters, established as suitable for a particular site in the gemstone material, is not necessarily that for any other site in the same gemstone, which does not allow the development of a universal protocol for laser marking. In fact, any working protocol for laser tagging should include real-time monitoring of the growth of each individual label in order to stop the laser labeling as soon as the label has the necessary overall dimensions. This aspect is important for entering labels that do not impair the appearance and grading of the quality of labeled gems.

Given the prior art noted above and the various problems and difficulties encountered in implementing related laser marking methods on or below the surface of gemstones, there is a need for a method and system that provides reliable, safe and controllable marking marks in the volume of gems such as diamonds. In addition, there is a need for a system that takes into account the stochastic nature and changes in labeling processes developed to date, together with the special physical properties of native diamonds during the formation of microstructures caused by the laser in them.

OBJECTS OF THE INVENTION

Therefore, the first objective of the present invention is to provide a method and device for the laser to enter constant point marks in the volume of precious stones, such as diamonds, at a predetermined depth below the surface of the plate and without causing any laser damage to the surface of the plate so that the inscribed marks were impossible to erase using any kind of surface treatment, and at the same time were very difficult for fraudsters to attack.

Another objective of the present invention is to provide a method for laser marking in the volume of diamonds, taking advantage of the presence of defects and impurities randomly distributed in the crystal lattice of native diamonds in order to initiate controlled growth of point marks by exposing the diamonds to laser pulses with a duration in the fentosecond range that carry energy much lower than the energy threshold for labeling in volume in other respects the ideal diamond material. Another objective of the present invention is to provide a method for safely labeling the highest purity diamond gemstones using a laser system that delivers laser pulses with energies high enough to cause structural changes in the volume of the ideal diamond crystal lattice.

Another objective of the present invention is to provide a method and device for laser marking in the volume of precious stones, such as diamonds, which would have sufficient versatility to allow the marking of precious stones with very different transparency and quality, having different facets and general sizes, and which the moment when they are labeled can be either separate or inserted into different types of frames.

Another objective of the present invention is to provide a method for inscribing laser point marks in the volume of precious stones, each sign should be small enough to remain undetectable when viewed with instruments that are commonly used by diamond sorters, so as not to impair the appearance, not to reduce the sorting and monetary value of the gem marked with the proposed method. On the other hand, another objective of the present invention is to develop the size and shape of the tags to make them machine-readable by a special optical reading system.

Another objective of the present invention is to provide a method for marking signs in a completely safe manner in the volume of precious stones, such as diamonds, and this method should provide due consideration to the stochastic nature of the formation of laser-induced marks in the volume of native diamonds having concentrations of defects and impurities that vary greatly from plot to plot in their volume.

Another objective of the present invention is to provide a simple, inexpensive and easy to use optical reading system based on the design of a conventional optical microscope and capable of providing images of point marks inscribed in the volume of the gemstone, and these images must have sufficient contrast to ensure reliable and automatic detection of the entire sign by image processing means.

Another objective of the present invention is to provide a method for encoding authentication data in the volume of precious stones, such as diamonds, by inscribing with the laser an unambiguously defined sign consisting of a very small number of point marks, and these marks must be sufficiently separated from each other so that the appearance, sorting The qualities and monetary value of the gemstones during labeling remained unchanged.

Another objective of the present invention is the creation of precious stones, such as diamonds, having a personalized, self-certification mark inscribed in their volume and preserving their original quality and monetary value.

These and other objects of the invention will become more fully understood from the following brief description of the invention and description of a preferred embodiment.

SUMMARY OF THE INVENTION

A method and device for marking signs consisting of a small number of opaque, dot marks in the volume of precious stones are proposed, moreover, these diamonds are preferably diamonds. The component mark marks are preferably engraved at one depth below the surface of a large faceted and polished facet (facet) of the diamond, and this facet is preferably a diamond plate (also called a pad). As a result, you can mark gems inserted into any type of frame. Each individual label is entered using a protocol specifically aimed at forming a label of the required size by exposing the gemstone surface to the smallest number of femtosecond laser pulses, with each pulse carrying energy that is usually much lower than the energy threshold to cause permanent structural changes in the ideal diamond crystal lattice . The depth at which the marks fit is controlled by focusing the femtosecond laser beam. In addition, precise focusing is selected for labeling in the volume of a gemstone product, while maintaining the integral density of the optical flux (fluence) (energy per unit area) on the surface of the product is much lower than the threshold for damage to the surface of the material.

In this case, labeling in volume is possible without causing any irreparable optical damage to the outer surface of the gem. The results of previous experimental studies reported by some groups regarding structural changes in diamond volume when exposed to a sequence of femtosecond laser pulses showed that these labels usually consist of a very different elemental form of carbon. Moreover, the microstructures created in it are practically opaque to light in the visible region of the spectrum. Amazingly, these opaque point marks can be made undetectable to the naked eye or by using an optical device having a 10x magnification, even if they are inscribed at depths only a few microns below the surface of the plate. It is enough to provide tight control along with a reasonable selection of some key parameters of the tagging process, such as pulse energy, effective numerical aperture of the focusing lens, laser pulse duration and spatial quality of the laser beam, to achieve point marks with a diameter not exceeding a few microns, preferably less than 5 microns .

The main aspect of the invention is that opaque point marks can be entered into the diamond volume using femtosecond laser pulses having an energy much lower than the threshold energy required to enter the highest quality diamond in the crystal lattice, i.e. in a crystal with practically no defects or impurities. When entering constant marks in the diamond volume, some caution is necessary, since the required integrated optical flux densities can cause damage on the surface of the product before the marks in the volume are entered. Exposure to a precious stone of high value by laser pulses having potentially “dangerous” energy levels can often be avoided by taking advantage of the presence of impurities and defects with a random distribution in the volume of native diamonds, including diamonds of the highest quality. These impurities and defects of various nature contribute to the creation of dark and opaque zones when exposed to femtosecond laser pulses with energies significantly lower than the threshold energy in other respects of an ideal material. The random spatial distribution of these defects and impurities in ordinary native diamonds determines the stochastic nature observed in previous attempts at labeling in a constant and reproducible manner in the volume of these stones. Another important aspect of the present invention is to take into account the spatially varying concentration of defects and impurities in native diamonds by developing a coding scheme in which the identification data is encoded at the relative positions of a small number of marks that form the mark.

Despite the typical diameter of point marks, which should be of the order of several microns, the opacity of these marks, when they are formed in diamond, allows them to be displayed with suitable contrast using an inexpensive optical reader. The reader device essentially contains a conventional microscope lens with a low numerical aperture, which transmits enlarged images of the entire engraved sign onto the plane of the sensor on a charge-coupled device (CCD) for capturing images. After that, the images are processed by the processing means to detect several marks that form the characters, followed by decryption of the identification data encoded in the characters. The illumination system of the optical reader increases the contrast of the images of the engraved marks, taking advantage of the lower faces (facets) of the gem, which act as effective light reflectors. The result of all the above aspects related to the optical reader is the simplicity of the design of the device, the ease of use by a user who is neither a gemologist, nor a microscope, and its low manufacturing cost, which makes it affordable for every jewelry store.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become clearer from the above detailed description of a preferred embodiment of the invention and its drawings. In these drawings:

figa, 1B and 1C are microphotographs showing labels engraved in the volume of different samples of diamond;

figure 2 presents a simplified block diagram of a complete system for tagging and authentication of precious stones;

figure 3 presents a block diagram showing the main blocks and components of a laser tagging system in accordance with a preferred embodiment of the present invention;

4 is a schematic view of various optical components and component blocks of a laser tagging system in accordance with a preferred embodiment of the present invention;

5 is a side view of an optical reader that provides images of a sign engraved in a gemstone volume in accordance with a preferred embodiment of the present invention;

6 is a side view of a diamond, in which two distinct marks are inscribed below the surface of the plate;

Fig. 7 is a top view of a diamond having a round brilliant cut and in which three distinct marks are inscribed below the surface of the plate and near the center of the plate;

on Fig presents a diagram showing the focusing of the recording laser beam into the volume of the gem;

Fig. 9 is a schematic view of a mark consisting of a set of five point marks in accordance with a preferred embodiment of the present invention;

on figa and 10B presents a block diagram of a sequence of operations performed by the authentication system of precious stones to enter a mark in the volume of the gem in accordance with the proposed method;

11 is a microphotograph showing an array of 25 point marks engraved in the volume of a sample of native diamond.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Gem Authentication System Brief

The various objectives of the present invention, indicated in the section "Objectives of the invention", relate to methods and devices that have found their main application in the system for authentication of precious stones using signs engraved in their volume. Figure 2 presents a simplified block diagram showing an embodiment of a gemstone authentication system. The heart of the system is the central processing unit (CPU) 20, which, in essence, is a computer that controls the operation of several remote devices connected to it by data communication channels 24. The main task of the CPU 20 is to manage requests for access to information stored in the database 22 , and also manage the entry of new data into the database registry. The data stored in the database 22 mainly consists of identification accounts associated with each labeled gem. The record includes a digital data stream that corresponds to the identification code engraved in the gemstone, together with other relevant information, such as a summary of the inherent properties of the gemstone (i.e. sorting report), its current owner, information about previous owners, the manufacturer of the gemstone stone, the place of extraction, where it comes from.

Remote devices that form part of the gemstone authentication system are divided into two main groups. The first group includes remote laser tagging stations 26, which operate under the control of the CPU 20. For simplicity, only two tagging stations 26A and 26B are shown in FIG. However, a valid authentication system will consist of a larger number of laser tagging stations, which could be conveniently distributed throughout the geographic area to be covered. The second group includes remote optical readers 28, which also operate under CPU 20. Only three remote optical readers 28A, 28B, and 28C are shown, but in practice these devices could be found in many places, including jewelry retail stores. police departments and diamond trading agencies. Optical readers 28 primarily serve to detect the presence of an authentication sign engraved in the volume of the gemstone being examined, and then transmit the input data (essentially an image) to the CPU 20 to correctly identify the gemstone. Each remote laser tagging station 26 also includes its own optical reader 28, designed to register each gem in the database 22 of the authentication system immediately after its tagging.

Description of a preferred embodiment of a laser tagging station

The roles played by the various component blocks of the preferred embodiment of the laser tagging station 26 will be better understood from the simplified block diagram shown in FIG. The arrows drawn in Fig. 3 by a double line show the laser beam, and the arrows by one line show the electrical connections necessary for various purposes, such as data exchange, transmission of command and control signals and power supply of some units. Each remote laser tagging station 26 contains its own control unit and processor 44, which can be implemented as a personal computer in a design for industrial use. The control unit and the processor 44 controls the operation of most of the components of the laser marking station 26 either by commands entered by the operator through the user interface 62, or by commands issued by the system software of the CPU 20 of the authentication system and transmitted via an external communication channel 24.

The laser system 42 creates a laser beam in the form of ultrashort pulses emitted in a pulsed periodic mode. For the action of the proposed method, it is necessary that the duration of the laser pulses be in the femtosecond range. More specifically, the pulse duration should not exceed several hundred femtoseconds; preferably, it should be less than about 100 fs. Illustrative examples of femtosecond laser systems are those that include a solid state amplification medium titanium-sapphire (Ti: sapphire) optically pumped by semiconductor laser diodes. These laser systems emit laser beams having a wavelength usually near the infrared region of the spectrum and, in particular, in the range from 750 to 800 nm. Femtosecond laser systems with a Ti gain medium: sapphire can be implemented as a single oscillator that generates laser pulses that carry energy in the nanoscale range and emitted with a repetition rate of usually tens of MHz (megahertz). However, it is possible to obtain laser pulses with energies up to several mJ by linking the laser output to a regenerative optical amplifier (positive feedback optical amplifier). One of the advantages of the proposed method is the ability to label precious stones with laser pulses having low energies of the order of several tens of nJ, so when using a laser system with a Ti: sapphire amplification medium, the use of a regenerative optical amplifier is absolutely not required. This advantage results in a significant simplification of the hardware coupled with a lower purchase price for the entire laser system. Since the efficiency of the laser tagging process depends on the spatial quality of the recording laser beam emitted by the laser system 42, the beam cleaning and conditioning unit 46 can perform spatial beam filtering. This unit also serves to adjust the spatial characteristics (ie, divergence and lateral beam size) of the laser beam in order to maximize the efficiency of the frequency conversion process performed by the frequency conversion unit 48. This process essentially consists in doubling the frequency of the optical center of the laser beam in this way that a laser beam with an initial wavelength of 775 nm can be converted into a beam with a wavelength of 388 nm. A frequency conversion unit 48 is optional for labeling certain gem materials and is based on second harmonic generation schemes well known to those skilled in the art. Figure 3 shows that the laser beam with the converted frequency then passes through laser tagging optics 52, which allows you to rigidly focus the laser beam at a certain depth below the input surface of the gem fixed in the installation site of the product 54. The control unit and processor 44 controls the movement of the installation site articles 54 using special drives of an electric motor 58 for marking in different places inside the gemstone.

The spatial characteristics of the recording laser beam are preferably monitored and controlled in real time by the control unit and the processor 44 using the data and images generated by the diagnostic unit of the recording beam 50. The diagnostic unit of the recording beam 50 is necessary in order to easily detect any change in the properties of the laser beam or any failure in the operation of the laser system. Both of these types of events could adversely affect the marking process or, in the worst case, cause irreparable damage to a gemstone exposed to a recording laser beam. Finally, one aspect of the present invention is the creation of a laser tagging protocol based on real-time monitoring of the growth of point marks in the volume of precious stones. This control is performed using images and real-time data provided by the process control unit 56. This unit uses some optical components of the laser tagging optics 52 to obtain the corresponding light signals from the area in which the tag is currently being expanded.

Figure 4 presents the layout, which shows the preferred placement of the optical components that form part of the various blocks necessary for the operation of the laser marking station in accordance with the proposed method. In this figure, bold solid lines show the optical paths of the laser beams propagating in this optical system. A small portion of the laser beam 70 emitted by the femtosecond laser system 42 is transmitted through a beam splitter plate 80, and then hits the photosensitive surface of the optical power meter 82. The reading from the power meter 82 is supplied to the control unit and processor 44 (not shown in this figure) to provide continuous monitoring of the laser 42 by measuring the average optical power of its output beam 70. The main part of the laser beam 70 is reflected by a beam splitter 80 and then passes through the cleaning unit and conditioning the beam 46. In a preferred embodiment, block 46 comprises two collecting lenses 84 and 86 with corresponding focal lengths and an iris 88 and a mechanical shutter 90, the opening of which is controlled remotely by the control unit and processor 44. The iris 88 is placed in the focal plane of the lens 84 , designed to provide spatial filtering, controlled by the diameter of the aperture. The mechanical shutter 90 provides a transmission of a pulse train of limited duration, which includes a predetermined number of laser pulses, this number being determined by the particular laser marking protocol currently being executed.

Then the spatially filtered sequence of laser pulses 72 is reflected by a flat mirror 92 with high reflectivity at the input aperture of the frequency conversion unit 48.

Frequency conversion is based on the generation of second harmonics, which occurs in some optical crystals without inversion symmetry, such as barium betaborate, lithium triborate, potassium titanyl phosphate or potassium primary acid phosphate. Then, the pulse energy of the laser beam 74 with the converted frequency is set by a predetermined value by a signal from the control unit and processor 44, which is supplied to an adjustable optical attenuator 94. This attenuator can be constructed, for example, in the form of a half-wave moderator plate installed at the rotation stage, followed by a polarizing beam splitter cube. This design is well known to specialists in this field. For the adjustable optical attenuator 94 to be configured in this way, it is necessary that the input laser beam 74 be linearly polarized.

After that, the laser beam having the necessary energy is reflected by a high reflectivity flat mirror 96 to the beam expander 98. Figure 4 shows a Galilean beam expander, which consists of an input scattering (negative) lens 100 and an output collecting lens 102. The focal lengths of the lenses 100 and 102 are selected such that the transverse size of the laser beam 76 is sufficiently enlarged to fill the entrance pupil of the focusing lens 118 without excessive clipping along the borders. Adequately filling the entrance pupil allows the focusing lens 118 to operate at its full numerical aperture. The main part of the transversely expanded laser beam 78 passes through the beam splitter plate 104 and is then reflected by the dichroic beam splitter plate 116 onto the entrance pupil of the focusing lens 118. It should be noted that the beam expander 98 and the focusing lens 118 are two main elements of the laser marking optics 52 shown in block diagram in figure 3.

The distance between the input aperture of the focusing lens 118 and the input surface of the gemstone 120 to be labeled is adjusted until the plane of the best focus of the focused laser beam is at the required depth in the volume of the gemstone 120. The gemstone 120 is fixed in the installation site of the product 54, which preferably contains holder 122, adapted to the size and shape of the gemstone, and the holder 122 is mounted on a stack of three linearly-movable electric tables 124A, 124B and 124C. Two of these moving tables move the gem in the transverse directions X and Y, and the third transverse table moves the gem in the Z direction parallel to the optical axis to precisely adjust the distance between the focusing lens 118 and the input surface of the gem 120. Linear moving tables 124A, 124B and 124C are controlled by a control unit and a processor 44 of the laser marking station using electric motor drives 58, as shown in FIG.

Figure 4 shows that part of the transversely expanded laser beam 78 is reflected by the beam splitter plate 104 to the diagnostic unit for the recording beam 50. In a preferred embodiment, this unit contains three optical channels, each of which is used to monitor a specific characteristic of the recording laser beam 78. The first the channel includes a CCD camera 110, which takes images of the transverse distribution of the beam intensity in the plane of the camera sensor, and the second channel includes a laser pulse counter 112. Tilt q, the third optical channel measures the time-averaged energy of the pulse by converting the reading optical power meter 114, given the reflection and transmission coefficients of beam splitters different plates in the path of the laser beam. The beam splitter plates 106 and 108 serve to direct parts of the recording laser beam 78 into different optical channels of the diagnostic unit of the recording beam 50.

In addition, FIG. 4 illustrates a preferred embodiment of a process control unit 56. The path of light that is detected by this unit is shown in FIG. 4 by dotted lines. Using optical devices, such as a camera 128 on charge-coupled devices and a fast photodetector 132, this unit is used to analyze images and light signals from a specific area, which is currently marked in the volume of the gemstone 120. For example, real-time images of this the zones can be captured by camera 128 on charge-coupled devices. With this solution, the focusing lens 118 is an integral part of the camera lens, which transmits an enlarged image of the corresponding zone to the plane of the sensor on the CCD of the camera 128. Images with adequate contrast can be obtained by appropriately highlighting the gem with a lighting device 134 during marking. In addition, to register the frequent pulses of light (plasma radiation) that are created when the diamond material undergoes local structural changes caused by the interaction of the material with intense ultra-high-speed pulses of the recording laser beam, you can use a fast photodetector 132. A band-pass optical filter 130 placed across the path of the beam light directed at fast photodetector 132 provides spectrally selective detection of light emitted when internal ructural changes. The beam splitter plate 126 directs parts of the light into two optical channels provided in the shown embodiment for the process control unit 56.

Within the scope of the present invention, various modifications of the above-described laser tagging device design are possible. For example, a frequency conversion unit 48 is optional for labeling in a gemstone volume, but in some cases additional control over the growth of point marks is provided by a recording laser beam with a shorter wavelength. In addition, spatial filtering by the beam cleaning and conditioning unit 46 is not required if the laser beam 70 at the output of the femtosecond laser system 42 has satisfactory spatial quality. The optical system shown in FIG. 4 can be modified to avoid using flat mirrors 92 and 96, although these mirrors are used to fine-tune the beam position. Finally, several lenses present in the optical system shown in FIG. 4, including the focusing lens 118, could be replaced with spherical mirrors.

Description of a preferred embodiment of an optical reader

FIG. 5 is a side view of a preferred embodiment of an optical reader 28 that forms part of the complete gemstone authentication system shown in FIG. 2. The design of the optical reader 28 is a relatively wide open imaging device in which a sign engraved in the volume of the gemstone 120 is displayed on the camera’s matrix sensor on the CCD 166. As a result, there is no raster scan of the probe laser beam over the surface of the gemstone 120. Microprocessor 186 receives the image data signals from the camera on the CCD 166 and then processes the image data files, and then sends them to the Central processor 20 of the authentication system displays on the communication data channel 24. The sign engraved in the volume of the gemstone 120 is displayed with adequate lateral magnification on the camera’s matrix sensor on the CCD 166, consisting essentially of a microscope lens 162 mounted on an extension tube 164. The microscope lens 162 is preferably a standard A serial lens designed for use with a 160 mm barrel. The exact length of the extension tube 164 is selected accordingly. It was found that enlarged images of signs having convenient overall dimensions fit well the size of CCD array sensors when selecting a microscope lens 162 that provides magnification in the range 10x - 20x. This range provides a satisfactory lateral resolution as well as a comfortable working distance.

Images with adequate contrast can be obtained from the camera on the CCD 166 using the reflected light illumination scheme, which provides bright field illumination of the marks engraved in the volume of the gemstone 120. Reflected light illumination basically means that the illumination light falls on the sample (in this case, gem 120) from the upper entrance surface (in this case, from the gem plate). In fact, a backlight illumination scheme was necessary to allow the optical reader 28 to work even with stones inserted in the frame, for which the backlight incident from the bottom of the sample is excluded. The backlighting below does not allow for the implementation of certain specific forms of precious stones. One aspect of the reflected backlight scheme provided for this preferred embodiment of the optical reader 28 is the annular shape of the backlight when it hits the gemstone plate 120. This light beam is shown in FIG. 5 by arrows 182. The diameter of the backlight ring in the plane the plates are chosen wide enough to avoid any direct illumination of the sign when the latter is in the central zone of the field of view of the optical reader. After entering the gem 120, the illumination light propagates downward and then is reflected internally upward in different directions by faceted and polished faces (facets) located on the lower part (pavilion) of the gemstone 120. In this case, the dot marks forming the sign are highlighted from below, usually appearing on the images as dark black spots on a bright background. The annular head 180 of the backlight device, the inner diameter of which corresponds to the microscope objective 162, delivers a beam of backlight 182 with an annular transverse shape. The annular heads of the backlight device are manufactured in different sizes by many suppliers of equipment for image visualization systems and machine vision.

In the preferred embodiment shown in FIG. 5, the backlight is generated by the broadband fiber optic backlight device 176, and the flexible fiber optic light guide 178 coupled to the output aperture of the backlight device transmits the backlight to the annular head 180 of the backlight. The backlight device 176, the fiber optic light guide 178, and the annular head 180 of the backlight device together form a complete backlight unit 174. Taking advantage of the reflected backlight scheme shown in FIG. 5, image contrast can be further increased by using means to prevent parts of the backlight beam 182 from getting into the area of the entrance surface of the gem, which lies directly above the engraved sign. To this end, a conical light screen 184 is coupled to the lower end of the microscope lens 162, which is designed to block any backlight 182 that would otherwise fall into the central part of the input surface of the gem. The aperture at the lower end of the conical light screen 184 is configured wide enough so that the microscope objective 162 can operate at its nominal numerical aperture.

The gem 120 to be examined by the optical reading device 28 is mounted in a holder 168 mounted on a support base 170. The holder 168 can be configured to provide accurate positioning of the gemstone 120, in which a sign engraved in the central area of the plate appears almost in the center of the field of view of the optical reader 28. To center the gemstone in the holder 168, you can provide a separate device (not shown in this figure), using, for example, a magnifier small increase containing graded eyepiece grid, two stage manual micrometer displacement and the base plate. After the gem is correctly centered on the ocular grid of the magnifier, the magnifier is removed, and the rest of the node is moved to the supporting base 170 until it rests against three separate control stops 172, of which only one is shown in Fig. 5. The holder can then be firmly fixed in position with a quick-release clamp 188. Finally, as part of the installation of the gemstone 120 in the holder 168, the plane of the gemstone plate is aligned with the horizontal reference surface of the holder assembly. This stage is necessary to ensure that the vertical position of the gemstone is accurately adjusted to easily bring the engraved sign into focus.

In a holder 168, such as the particular holder shown in FIG. 5, only individual, non-inserted gemstones can be attached. However, one of skill in the art can easily make changes to some parts of the holder so that signs engraved in precious stones in the frame, such as precious stones inserted in rings, earrings, pendants, pendants, dangling parts of earrings and bracelets, can be read. In this case, the optical reader 28 is usually used with a set of holders 168, suitable for precious stones inserted in different frames.

The optical reader assembly 28 can be housed in a variety of ways. For example, all the components shown in FIG. 5, including microprocessor 186 and associated electronics, can be placed in one protective case or cabinet. Preferably, the cabinet should have a pleasant appearance suitable for an environment such as a counter of jewelry retail stores. A hole in the door made on the front side wall of the cabinet allows the operator to insert the gemstone holder 168 into the assembly to set the gemstone 120 exactly aligned with the optical axis of the optical reader. In the front side wall of the cabinet there is a user interface consisting of a liquid crystal display and a control panel. In addition, the node optical reader 28 can be packaged in the form of a manual remote sensing head connected to the control unit and interface. This hand-held device contains a microscope objective 162, an extension tube 163, a CCD camera 166, and a full illumination device 174, all of which are small-sized so that they can be packaged in a convenient device that can be held by hand. For example, the illumination device 174 can be made in the form of a compact annular illuminator in which light is generated by a matrix of solid-state LEDs. In addition, the special microscope objective 162 can be designed so that it displays an object in a plane closer than the aforementioned standard distance of 160 mm. The manual construction has the advantage that it does not require the use of a gemstone holder 168 and associated parts 170, 172 and 188, since the gemstone 120 is simply brought into contact with the front end of the probe head. For this purpose, the front end contains a flat plate made of a solid transparent material on which a gemstone plate is placed with close contact. A flat plate is used to place the gemstone plate at the correct working distance from the front end of the microscope lens. Alignment marks engraved on a flat plate help center the gem relative to the optical axis of the reader. In this embodiment, the probe head is held in one hand and the gemstone 120 in the other hand is either with tweezers in the case of a single stone, or by the rims inserted in the rims of the gemstones.

Labeling in gem volume

Fig.6 is a side view of the gemstone 120, in the volume of which two excellent point marks are engraved, indicated by the same position 148. In particular, Fig.6 shows a diamond having a round diamond cut. The plate of this diamond gemstone is the upper horizontal flat surface 140, upon which a recording laser beam is incident upon marking. A diamond having a round diamond cut also has a crown 142 and pavilion 146; both of these parts consist of several faces (facets), not shown in this figure. The girdle 144 is a peripheral girdle separating the crown 142 from the pavilion 146.

One important aspect of the proposed method is the labeling of point marks having an adjustable size in the volume of a gem, such as diamond. It is clear that when marking in volume, the surface of the fall (i.e. plate) of the gemstone, as well as part of the volume of the gemstone material located along the internal path of the recording laser beam, should in no case be changed. In other words, laser-induced structural changes that result in the formation of permanent marks should only begin in a thin layer at a depth d below the gem plate 140, as shown in FIG. 6. The thickness of this imaginary thin layer is determined by the overall accuracy of the process when marking at a nominal depth d below the plate 140. For simplicity, it is usually desirable that all marks 148 lie at the same depth, as this helps to ensure that the entire set of marks is in exact focus in the images, captured by the optical reader 28. The tags 148 are distributed in a thin layer in accordance with the pattern, which depends on the symbology (coding scheme) selected for encoding the authentication data, as well as on the specific identification code attributed to this gem. For example, FIG. 7 is a plan view of a diamond having a round brilliant cut, which shows three distinct marks 148 forming a mark. Rectangle 150, drawn with dashed lines (with short dashed lines), limits the outer contour of the field of view in the subject plane of the optical reader 28, made in accordance with a preferred embodiment of the present invention. For this embodiment, shown in FIG. 5, it is necessary that the marking of the mark is performed in the central area of the plate 140.

In another embodiment of the proposed method, marks could fit within different depths of the gemstone 120 within the meaning of the present invention, thereby leading to the insertion of three-dimensional characters. Compared to their two-dimensional counterparts, three-dimensional signs have the advantage of greater secrecy, since the components of the sign of the mark cannot be brought into focus at the same time, if you look at them through an optical device, the depth of field of the spatial image of which is shorter than the range of depths in which the components of the mark of the mark are engraved. On the other hand, the great difficulty in detecting three-dimensional characters means that the design of the optical reader 28, shown schematically in FIG. 5, needs to be modernized so that planes at different depths in the gemstone volume can be displayed. Such an upgrade could be accomplished, for example, by controlled vertical movement of the holder 168. In this case, the images would be sequentially recorded by the camera on the CCD 166 during the vertical movement of the gemstone 120. The entire sign could then be restored by combining a subset of the images in which each individual the mark is brought into sharp focus, while maintaining their relative positions unchanged. The resulting composite image could then be sent to the CPU 20 for subsequent decryption of the sign. This additional embodiment of the optical reader 28 has the advantage of eliminating the need to enter marks at very precise depths in the volume of the gemstone 120.

6 and 7, the relative sizes of the marks 148 are shown to be very enlarged, since in practice they should remain undetectable when looking at the plate with the naked eye or through an optical device having a 10-fold increase. It is even very necessary that the tags be difficult to detect when viewed without any preliminary search point through binocular microscopes currently used in gemology. The intrinsic opacity of the constituent material of the engraved marks 148 makes it particularly difficult for them to be hidden if the aforementioned visual aids are used. The key to the secrecy of tags is to perform a laser tagging operation that enables subsurface inscribing of tags having common individual sizes not exceeding about 5 microns, preferably less than 2 microns.

Gemstone recording laser beam focus control

One important aspect of inscribing a laser dot mark with a diameter of only a few microns relates to focusing the recording laser beam into the volume of the gemstone 120. Focusing the recording laser beam into the volume of the gemstone 120 is shown schematically in FIG. Arrows 152 show the general outer contour of the optical intensity distribution of the recording (transversely expanded) laser beam 78 (see FIG. 4), which propagates along the optical axis 156 and enters the entrance pupil of the focusing lens 118. Arrows 154 show the outer contour of the recording laser beam exiting from the lens 118, rigidly focused on the volume at a depth d below the plate 140 of the gemstone 120.

In order to achieve the necessary beam characteristics in the volume of the gemstone 120, it is necessary to carefully select the numerical aperture of the lens 118, which is an indicator of the angular divergence of the beam emerging from this optical component. On the one hand, with an increase in the numerical aperture of the lens, the diameter W _{F of the} laser beam intensity profile in the plane of best focus becomes smaller. This trend is observed in a mode in which the focused laser beam 154 is not strongly distorted by spherical aberrations arising from its propagation through the various optical elements of the lens 118. In addition, increasing the numerical aperture of the lens helps to reduce the risk of laser damage to the surface of the plate 140 caused by this. This is the result of the diameter W _{S of} the beam intensity distribution in the plane of the plate 140, which can be made significantly larger than the corresponding profile diameter W _F beam intensities in the plane of best focus. As a result, the integrated density of the optical flux (fluence) (energy per unit area) in the plane of the plate 140 may be much lower than the fluence necessary to initiate the development of a point mark 148.

On the other hand, the use of a lens 118 with a higher numerical aperture results in a smaller (and possibly inconvenient) working distance S shown in FIG. 8, and the focused laser beam can significantly deteriorate with any residual optical roughness present on the polished surface of the plate 140 . Furthermore, in the evaluation of the minimum transverse dimension W _F beam in best focus plane is necessary to consider the influence of spherical aberration caused by the passage of the writing laser beam at the lens 118. A practical solution is to focus the lens 118 having a focal length in the range of 5-10 mm and a numerical aperture of 0,35-0,55. Due to the very hard focusing of the recording laser beam, in order to obtain the plane of best focus at the required depth d below the plate 140 together with the required beam spot size W _F in this plane, it is necessary to perform calculations to construct the beam path, taking into account the exact optical design focusing lens 118. The depth d is preferably set in the range of about 200 to 700 microns. Inserting marks at a greater depth below the surface of plate 140 provides greater concealment of marks. On the other hand, a larger propagation path of the recording laser beam into the gem material increases the likelihood of the beam being disturbed by natural inclusions and other types of inhomogeneities present in the material.

Description of a preferred embodiment of a coding scheme

Above, some aspects of the method of engraving point marks having a total size of preferably about 1 μm in the gemstone volume are described in detail with the aim of making each individual mark almost invisible using visual means commonly used in the art. Unfortunately, it is easy to understand that if a mark consists of an excessively large number of opaque point marks distributed over a limited area, it can become easily visible. Accordingly, another important aspect of the present invention is a method for encoding machine-readable identification information in characters formed from only a small number of individual tags.

Figure 9 presents a schematic view of the sign 198, made according to the preferred proposed coding scheme. Sign 198 contains five labels, which, in accordance with their specific roles in the coding scheme, can be divided into two groups. Thus, the first group of marks, indicated by 200A, 200B and 200C, form the corners of a geometric figure, the purpose of which is to allow accurate recognition of the sign 198 by computer software that processes the images transmitted by the proposed optical reader 28. The specific geometric figure shown in FIG. 9 is a triangle whose sides are drawn in dashed lines. By including additional marks in the first group, other geometric shapes can be constructed within the spirit of the preferred coding scheme in accordance with the present invention. The second group includes two labels, indicated by 292A and 202B. These tags are used exclusively for encoding identification data. Additional labels can be included in the second group. In a preferred coding scheme, numerical data that uniquely identifies a gemstone is encoded at the positions of points 202A and 202 B. These positions are expressed in pairs of spatial coordinates (X ₁ , Y ₁ ) and (X ₂ , Y ₂ ), respectively. In order to encode the identification data in order to increase the number of different combinations allowed by the encoding scheme, some specific attributes of the triangle shown by the dashed line are used. In the example shown in Fig. 9, to create the complete identification code attributed to the gem, the values of the two internal angles α and β are added to the pairs of spatial coordinates mentioned above. The full numerical identification code obtained from this character 198 can be expressed as a data stream (X ₁ , Y ₁ , X ₂ , Y ₂ , α, β) consisting of six elements. To increase the number of excellent identification codes, this data stream can be lengthened by including spatial coordinates associated with additional coding marks.

At first glance, the presence of coding marks 202A and 202B will not allow reliable machine recognition of sign 198, since the five marks shown in this figure can be used to construct a large number of different triangles. Recognition software can eliminate this potential difficulty. To do this, he just needs to give the command to take into account the triangle having the longest side - in this case, side 208 of length L, as shown in Fig. 9. This means that all equilateral triangles are excluded from the preferred coding scheme. The exclusion of equilateral triangles also gives a recognition scheme that is invariant when the sign 198 is rotated on images obtained from the optical reader 28. Rotationally invariant recognition of the sign 198 is important because the label 200C, which forms the left end of the largest side 208 of the triangle, determines the origin of the rectangular XY coordinate system in which the positions of the coding marks 202A and 202B are determined. The rectangular X-Y coordinate system associated with the sign 198 is shown in FIG. 9 by the X axis 204 and the Y axis 206. Linking the spatial coordinates of points 202A and 202B to the position of point 200C ensures that the decoding process is also invariant when moving the sign. This advantageous property implies that the sign 198 does not need to be placed in a specific place on the surface of the gemstone plate. In addition, the sign does not need to be accurately centered in the output signal of the images from the optical reader 28.

The decoding of the identification code encoded in sign 198 is made invariant when scaling by expressing the spatial coordinates of the coding marks 202A and 202B with respect to the length L of the largest side 208 of the triangle. In this case, the individual spatial coordinates X ₁ , Y ₁ , X ₂ and Y ₂ are given by values internally limited by an interval from 0 to 1. Implementation of the recognition process, which is invariant when scaling images, is very effective if these images are captured by various optical reading devices 28 equipped with 162 microscope lenses that do not necessarily give the same magnification. In addition, the exact physical length L of the side 208 does not affect character recognition and its subsequent decoding. In practice, the length of the largest side 208 of the triangle is selected so that the entire sign 198 can always be completely enclosed in the field of view on the subject plane of the optical reader 28, regardless of how the sign is rotated relative to the contour of the field of view. However, in some cases, the overall dimensions of the sign must be kept relatively small, since it is extremely preferable that the entire area bounded by the outer contour of the sign does not have any natural inclusion that could be detected in images captured by the optical reader 28. The absence of any sign or inclusions are especially important in cases where these inclusions can be very reminiscent of engraved marks. At the same time, there is no need to filter them from images with recognition software before starting recognition of the sign.

In another embodiment of the optical reader 28, the magnification of the microscope lens 162 could be accurately calibrated so that the actual length L of the largest side 208 of the triangle, which serves to recognize the sign 198, can be accurately measured. The measured value of L can then be included as the seventh element in the stream data (X _1, Y _1, X _2, Y _2, α, β), which is a numerical representation of the identification code, encoded in the symbol 198. The addition of the measured value L as part of the identification codes yields Res ltate a significant increase in the number of different combinations allowed by the encoding scheme.

In the preferred coding scheme, labels 202A and 202B are always inside the triangle bounding the sign 198, so that the range of acceptable values for their spatial coordinates covers only a limited part of the maximum interval covering values from 0 to 1. The change interval for each individual coordinate X ₁ , Y ₁ , X ₂ or Y ₂ in the example shown in Fig. 9, depends, in fact, on the preliminary selection of a pair of angles α and β, which determine the specific shape of the triangle. In addition, in order to avoid any confusion between identification codes that differ only in the value of one coordinate, only discrete values are allowed for any given coordinate. In practice, the increment between two consecutive values allowed by the spatial coordinate is dictated by the general resolution of the optical readout circuit. This resolution depends on a number of factors, such as the intrinsic size of each inscribed mark, the plane resolution (or resolution) of the microscope objective 162 of the optical reader 28, the sizes of the sensitive photocells of the camera matrix sensor on the CCD 166, and the ability to capture the images in sharp focus. For example, to ensure that two adjacent marks separated by an increment are always clearly visible on images transmitted by an optical reader, for the allowed spatial coordinates of the marks having a diameter of 1 μm, an increment of approximately 4 μm can be set. This means that if the triangle bounding the sign 198 had the largest side, say, L = 300 μm, the X coordinate of the coding marks could take a maximum of 75 different values. In the above example, the number of acceptable values for each coordinate will actually be significantly lower than 75, since the upper boundaries of the intervals in which coordinates can change are determined by the other two sides of the triangle. This is especially evident for the coordinates Y ₁ and Y ₂ related to the vertical positions of the coding marks 202A and 202B in FIG. 9.

Laser marking method in precious stones with benefit from the presence of internal defects and impurities

The preferred sequence of operations when inscribing laser marks in the volume of the gemstone is shown in the flowchart shown in figa and 10B. This sequence of operations is carried out by exchanging messages between the CPU 20 of the gemstone authentication system shown in FIG. 2 and the remote laser tagging station 26. In a first step 220, an identification code is generated by the central processor 20 in accordance with the requirements and rules specified in the encoding scheme, used in the authentication system. Then, at step 230, the CPU 20 is provided with access to the database 22 to check whether the newly created identification code is assigned to any previously labeled gemstone. If it is determined in step 240 that it is reserved, it is immediately changed in step 250 and then checked again in step 230 until a valid identification code is finally received. Then, at step 260, based on the selected identification code, a corresponding character pattern is created in accordance with the preferred symbolism, such as that shown in FIG. 9. The construction of a picture for a sign consists mainly in establishing the relative spatial position of each of the various marks, which forms part of the sign, so that the identification code is correctly encrypted in the figure. The mark pattern is then converted into a sequence of machine instructions, which is transmitted to the laser tagging station 26 so that the tagging process can be carried out at step 270. The marks are engraved in sequential order, and the gem is kept stationary when each individual mark is entered. After the successful completion of the marking operation of any given mark with electric-powered linear-movement tables 124A, 124B and 124C of the product installation unit 54 (see Fig. 4), the gemstone 120 is moved until the next marking place exactly matches the optical axis of the recording laser beam.

One of the novelty features of the present invention is that laser-induced structural changes in the gem material leading to the growth of an opaque spot are triggered by defects or impurities present in the bulk of the material where the recording laser beam reaches its smallest transverse dimension or, equivalently, its maximum integrated optical flux density. Native diamonds usually contain a wide variety of invisible structural defects and impurities, most of which are impurity atoms, such as nitrogen, hydrogen and boron atoms, with nitrogen atoms being the most common. The initiation of the labeling of point structures from internal defects begins the process of labeling with femtosecond laser pulses that carry energy far below the threshold energy necessary to create structural changes in other respects in the ideal diamond material. As a result, a recording laser beam can be emitted by lasers with a solid-state amplification medium titanium-sapphire (Ti: sapphire) without the need for any subsequent optical amplification of the laser pulses. In addition, when using laser pulses having “safe” levels of integrated optical flux density in a plane that matches the plate, the risks of optical damage to the gemstone plate are dramatically reduced.

At the same time, one of the serious drawbacks of initiating the construction of opaque labels with naturally occurring defects and impurities is due to the random spatial distribution of these defects along with their concentration, which varies significantly from site to site in the same gem. In addition, gemstones of very high quality, such as sorted as internally defective, often have zones in their volume that have virtually no “necessary” defects, thereby requiring increased energy levels and (or) more laser pulses. In practice, the laser tagging protocol will typically include a gradual increase in the pulse energy before initiating label growth. The maximum allowable energy will be determined by the particular laser system used in the tagging station, and this energy could exceed the threshold energy to cause structural changes in the volume of the ideal gem material. In this case, the laser tagging protocol will provide for the possibility of marking in a section that does not have any defects or impurities. However, as shown in FIGS. 1A and 1C, using elevated energy levels increases the likelihood of initiating the growth of unwanted marks all along the path of the recording laser beam inside the gem. In addition, the maximum pulse energy allowed for a reliable and safe fit in gemstones such as diamonds may be limited by non-linear optical effects such as self-focusing, especially when using a focusing lens with a lower numerical aperture.

The proposed method allows you to get rid of the disadvantage caused by a random distribution of internal defects and impurities in the gem to be labeled by real-time monitoring of the growth of each individual label. In the event that any given mark is unsuccessful in step 280 due to apparently no defects in the volume around the focused recording laser beam, the failure event is reported to the central processor 20 and a new position for engraving the mark is determined, as shown in step 290 in FIG. 10A . At step 300, an identification code calculated in accordance with the newly determined position of the mark is calculated, then validity is confirmed at steps 230-240. Then, at step 270, laser marking is started in a new position. The operations are repeated until the label can be successfully entered, and the whole method is applied to the entire set of labels forming the sign. As a result, the identification code and the associated mark obtained at the end of step 310 of the successful tagging operation may differ significantly from those created at the very beginning of the tagging operation, especially when engraving in precious stones having very high transparency.

11 is an optical microphotograph showing a 5 × 5 square array of point marks inscribed in a controlled manner at a depth of about 300 μm below the native diamond plate in accordance with an exemplary laser tagging protocol. The selected protocol provided for the supply to the gem of the first pair of laser pulses with a wavelength of 775 nm and a duration of about 150 fs, measured directly at the output of the laser system. These two laser pulses were separated by a time interval of 1 ms. The laser pulses were focused by a lens consisting of a single aspherical lens having a numerical aperture of 0.5 for the transverse distribution of the beam intensity with a diameter of about 8 mm in its entrance pupil. The pulse energy of the recording laser beam, measured at the entrance pupil of the focusing lens, was slightly less than 1 μJ. Successful pairs of laser pulses with characteristics identical to those given above were sent to the sites until the final diameter of each mark reached about 3-5 microns. During labeling, to control the gradual growth of the sign, each area was displayed on the camera at the CCD. A label of a suitable diameter could be successfully entered in each of 25 different sections of this particular sample of diamond. The number of pairs of pulses required to enter a label of a suitable diameter varied from site to site, but for the indicated pulse energy it never exceeded five. The depth at which the marks were inscribed changed somewhat from site to site, so it was impossible to catch the images of all the marks in sharp focus on the same microphotograph. The distance between adjacent marks shown in FIG. 11 is about 50 μm. The array of point marks covers an area of approximately 250 × 250 μm, which corresponds to a typical total size of the sign engraved in accordance with the proposed method.

Returning to FIG. 10B, at step 320, an optical reader 28, which is part of the laser tagging station 26, captures a machine-readable image of a sign again engraved in the volume of the gem. In step 330, an identification code is extracted from the sign image by the central processor 20, and in step 340 this identification code is compared with the identification code that was currently valid at the end of the tagging operation. Theoretically, both of these identification codes should be identical, but possible malfunctions or malfunctioning of the hardware of the laser tagging station 26 can lead to differences between the required identification code and the one that corresponds to the sign actually engraved in the volume of the gemstone 120. When this event occurs, the central by process 20, at step 350, a warning is issued to the operator of the laser tagging station. Thereafter, in step 360, the newly engraved gemstone is registered in the authentication system by recording in the database 22 the identification code extracted in step 330, together with some other identification data. A specific image can be included in the data packet that is entered into the database, which was captured by the optical reader 28, and then processed by the CPU 20 to extract the gemstone identification code. Finally, at step 370, an authentication certificate is printed out and the process is completed.

Although the preferred embodiment of the invention has been described above with its various aspects, this description should be considered as an illustration of an embodiment of the invention, and not as a description of its intended scope. The volume will become clearer from the disclosure as a whole.

Claims (40)

1. A method for adaptively controlling the creation of characters in a gemstone sample volume using a sequence of pulses in the femtosecond range focused below the surface of the specimen, said characters identifying the specimen without affecting the specimen surface and making the characters invisible at 10x magnification, which includes the following stages:
the stage at which the distinguishing features of the characters to be created are defined;
the stage at which the specified labeling protocol is performed for the indicated sequence of laser pulses using parameters selected from the group including wavelength, pulse duration, number of pulses, repetition rate, pulse energy, numerical aperture of the focusing optics and target coordinates;
the stage at which during the execution of the specified Protocol control the creation of these characters; and the stage at which the further execution of the specified protocol is stopped if, when the specified control, the manifestation of the indicated distinctive features by the indicated signs is detected. 2. The method according to claim 1, characterized in that these distinctive features are selected from the group including shape, size, optical properties and location in the sample. 3. The method according to claim 2, characterized in that the indicated distinctive features include the size, and the specified sizes take a size that does not reduce the commercial value of the specified sample. 4. The method according to claim 2, characterized in that said control stage includes controlling the creation of said characters during the execution of said protocol and, based on the result of said control, changing at least one of said parameters. 5. The method according to claim 1, characterized in that in the specified labeling protocol include several sets of these parameters for the sequential execution of these sets. 6. The method according to claim 5, characterized in that each sequential set includes a change in pulse energy in excess of the pulse energy in the previous set in the sequence. 7. The method according to claim 1, characterized in that the control is performed using an optical imaging device to assess the presence of these distinctive features. 8. The method of claim 7, further comprising the step of using a fast photodetector to detect light pulses signaling structural changes in said sample and a light filter. 9. The method according to any one of claims 1, 5 to 8, characterized in that a diamond is taken as the specified sample of the precious stone. 10. Adaptive control method for controlling the application of signs in the volume of a gemstone using a sequence of pulses in the femtosecond range focused below the surface of the specified sample, with the indicated signs the specified sample is identified without affecting the surface of the sample, and these signs are made invisible at 10x magnification which includes the following steps:
the stage at which, under the control of the processor, an identification code is created for binding to the specified sample;
the stage at which the distinctive pattern for several characters is determined, corresponding to the specified identification code;
the stage at which, under the control of the specified processor, the labeling protocol is performed for the indicated sequence of laser pulses by applying the indicated pulses in order to sequentially create each of the indicated characters in accordance with the specified distinctive pattern; and
the stage at which the specified processor is controlled in such a way that if, when the specified protocol is executed, one, and not all, of the indicated characters is created in accordance with the specified accusatory pattern, the specified processor creates a new identification code corresponding to the new distinctive pattern, consistent with those of of said characters that have been successfully created, and if necessary, by means of a processor, execute a tagging protocol to create additional characters in order to complete Ia said new distinctive pattern. 11. The method of claim 10, further comprising a step in which the creation of each of the several characters is in turn controlled. 12. The method of claim 10, further comprising the step of: upon completion of the creation of said distinctive character pattern or said new distinctive character pattern, said identification code or said new identification code, as the case may be, is recorded in the database. 13. The method according to p. 12, characterized in that the specified stage of creating an identification code further includes a stage in which the specified database checks whether the specified identification code, creates an additional identification code, and checks the specified database if the specified additional identification code. 14. The method according to any one of paragraphs.10 to 13, characterized in that the diamond is taken as the specified sample of the precious stone. 15. A gem authentication system comprising:
a marking device for applying patterns of signs in the volume of precious stones using a sequence of ultrashort laser pulses focused below the surface of the precious stones, and these signs are invisible at 10x magnification;
a database that uniquely associates an identification code with each of these patterns of characters;
several readers associated with several remote locations to detect the specified patterns of characters; and
a processor structurally configured to communicate with said tagging device, said database, and said reading devices. 16. The system of claim 15, wherein said database further associates said identification code with data relating to one of said gemstones. 17. The system according to clause 15, wherein the specified processor is designed to transmit commands to the specified tagging device to apply one of the indicated patterns of signs in one of these gems. 18. The system of clause 15, wherein said processor is structurally designed to communicate with said remote locations, for receiving information from one of said remote locations that identifies one of said character patterns, for searching and retrieving from said database data and transmitting said data to one of said remote locations. 19. The system of clause 15, wherein said tagging device also includes an optical imaging device for evaluating the creation of characters in real time, and the specified processor is structurally designed for
control the operation of the specified tagging device depending on the state of the creation of characters;
fitting the parameters of the specified sequence of laser pulses in accordance with the assessment of the creation of characters in real time; and
messages to the specified database about the successful drawing of the character pattern. 20. The system according to claim 19, characterized in that said processor is structurally designed to select an alternative pattern of characters in case the specified pattern of characters is not successfully applied to said gemstones. 21. The system according to claim 20, characterized in that said processor is structurally executed for consultation with said database when selecting said alternative character pattern. 22. An adaptive control method for controlling the creation of characters in a gemstone sample volume, wherein said characters are identified by said characters without affecting the surface of the sample, and said characters are made invisible at 10x magnification, which includes the following steps:
the stage at which the labeling protocol is set for the labeling system with ultrashort laser pulses, and several specified parameter sets are included in the specified protocol, and parameters are selected in each set, which are selected from the group including wavelength, pulse duration, number of pulses, repetition rate, energy pulse, numerical aperture of focusing optics and target coordinates;
the stage at which an attempt is made to create a character by executing the first set of parameters that are determined by the specified protocol;
the stage at which it is evaluated whether the character was created using the specified first set of parameters;
a stage in which, if a character has not been created, an attempt is made to create a character in accordance with a second set of parameters that are specified by the specified protocol. 23. The method according to item 22, wherein the diamond is taken as the specified sample of the gemstone. 24. A device for applying marks in the amount of gemstones, said signs identifying said gems and being invisible at 10x magnification, comprising:
a laser system for focusing laser pulses with a duration of less than 100 femtos at selected depths below the surface of the gem;
storage means comprising a labeling protocol containing parameters for operation of said laser system, said parameters being set and selected from the group including pulse duration, number of pulses, repetition rate, pulse energy and numerical aperture;
a Central processing unit (CPU) for controlling the operation of the specified laser system in accordance with the specified labeling protocol; and
a process control unit for evaluating the creation of each character. 25. The device according to paragraph 24, wherein the specified CPU controls the operation of the specified laser system in accordance with the assessment - the process control unit - create the specified sign. 26. The device according A.25, characterized in that the specified protocol includes several sets of these parameters for sequential execution, and the specified CPU operates in such a way as to ensure the execution of the second set of these parameters, if the first set of parameters does not cause the creation of a character. 27. The device according to p, characterized in that the specified second set has a different pulse energy than the specified first set. 28. The device according to paragraph 24 or 27, characterized in that said process control unit comprises an optical imaging device. 29. The device according to paragraph 24 or 27, characterized in that the process control unit comprises a filter in combination with a fast photodetector. 30. The device according to paragraph 24 or 27, also containing a diagnostic subsystem of the recording beam, comprising at least one optical channel selected from the group including image pickup means, a pulse counter, and an optical power meter. 31. The device according to paragraph 24, also containing a database for unambiguously linking data for each of several gemstones with patterns of signs recorded in said gemstones, characterized in that said CPU ensures the execution of the indicated tagging protocol with target coordinates determined by one of specified character drawings. 32. The device according to p. 26, also containing a database for unambiguously linking data for each of several precious stones with patterns of signs recorded in these precious stones, characterized in that the CPU is structurally designed to perform the specified tagging protocol with target coordinates defined one of the indicated patterns of characters, and if during the execution of the specified protocol one character of each pattern is not created, to communicate with the specified database for searching and retrieving an alternative pattern, the character in. 33. The device according to any one of paragraphs.24 to 27, characterized in that said precious stones are diamond. 34. The device according A.25, characterized in that the CPU is used to change the parameters by which the specified work is controlled, in accordance with the assessment - the process control unit - the creation of these characters. 35. A gemstone specimen containing subsurface marks for identifying said specimen, said subsurface marks being invisible at 10x magnification and applied by the method according to claims 1, 10 or 22. 36. The sample according to clause 35, wherein the specified sample is a diamond. 37. A gemstone specimen containing several subsurface marks marked in it to identify the specified specimen, characterized in that said several marks are encoded in the spatial arrangement of localized zones, and in that said localized zone exhibits optical characteristics different from those of the environment of said localized zone, and these signs are invisible at 10x magnification. 38. The sample according to clause 37, wherein the first subset of these several subsurface characters defines a coordinate system, and the second subset of these several subsurface characters encodes the identity of the specified sample. 39. Adaptive control method for controlling the application of signs in the gemstone sample volume, wherein said characters are identified without specifying the surface of the specimen, and the characters are made invisible at 10x magnification using a sequence of pulses in the femtosecond range focused below the surface of the specimen which includes the following steps:
the stage at which the distinguishing features of the characters to be created are defined;
the stage at which, under the control of the processor, an identification code is created for binding to the specified sample;
the stage at which the distinctive pattern for several characters is determined, corresponding to the specified identification code;
the stage at which the labeling protocol is performed for the indicated sequence of laser pulses by applying the indicated pulses in order to sequentially create each of the indicated characters in accordance with the specified distinctive pattern using parameters that are selected from the group including wavelength, pulse duration, number of pulses, frequency following, pulse energy, numerical aperture of the focusing optics and target coordinates;
the stage at which during the execution of the specified Protocol control the creation of these characters;
the stage at which the specified processor is controlled in such a way that if, when the specified protocol is executed, one, and not all of the indicated characters are created in accordance with the specified distinctive pattern, the specified processor creates a new identification code corresponding to the new distinctive pattern, consistent with those of the indicated characters that have been successfully created, and if necessary, the tagging protocol is executed by the processor to create additional characters in order to complete niya specified new distinctive pattern;
the stage at which the further execution of the specified protocol is stopped if the manifestation of the indicated distinctive features by the indicated signs is detected; and
the stage at which, after completion of the creation of said distinctive character pattern or said new distinctive character pattern, said identification code or said new identification code, as the case may be, is recorded in the database. 40. The method according to § 39, characterized in that the diamond is taken as the specified sample of the precious stone.

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